1. Field of the Invention:
This invention relates to a hydraulic circuit system for use in hydraulically operated vehicles such as, for example, a hydraulic power shovel, and more particularly to a hydraulic circuit system for controlling a slewing motor associated therewith.
2. Description of the Prior Art:
As for the system for controlling the slewing motor associated with a hydraulic power shovel, there have so far been employed two kinds of modes for controlling the gyratory unit; one is a swivel braking mode wherein when the control lever is located at its neutral position the brake gear is automatically actuated hydraulically or mechanically to forcibly stop the gyratory unit, and the other is a free swivelling mode wherein when the control lever is located at its neutral position the gyratory unit continues to turn by the influence of inertia, and arrangement is made such that the gyratory unit cannot be stopped without operating the control lever reversely or actuating the brake gear separately.
In general, the swivel braking mode, in particular, hydraulic braking mode has posed a problem on fine regulation operability. In effecting underground pipeline laying work which requires suspension of pipes, operability to conduct fine regulation is needed, however, the swivel braking mode tends to cause shock.
Whilst, the free swivelling mode is advantageous in that, when lifting a pipe, the gyratory unit turns of itself to allow the sling wire connected between the leading end of a boom of the power shovel and the pipe and extending diagonally to be tensioned vertically by the force of gravity so as to reduce the swing of the pipe being lifted, so that the pipe lifting work is conducted easily and safely.
In view of the foregoing, it is desirable for the operator to make it possible to use selectively the above-mentioned two kinds of controlling modes depending on the nature of the work to be carried out.
FIG. 4 illustrates a prior art example of hydraulic circuit system arranged such that the above-mentioned two kinds of controlling modes can be used selectively or switched over from one to another mode and vice versa.
In this prior art example, a bypass circuit "d" is provided which bypasses a counterbalancing valve "c" interposed between a slewing motor "a" and a directional control valve "b" adapted to control the slewing motor for switchover thereof. The bypass circuit "d" includes a changeover or control valve "e" which has on and off positions. The arrangement is made such that, when changing over the control valve "e" to its ON position or OFF position by means of a switch "f" and a solenoid valve "g", the slewing motor "a" can be switched over to either the swivel braking mode or the free swivelling mode. Further, the arrangement is made such that when the directional control valve "b" is located at its neutral position and circuits for rotating the slewing motor forwardly and reversely are interconnected.
The above-mentioned prior art system has been disadvantageous in that in case of the free swivelling mode with the control valve "e" changed over to its ON position, the slewing motor "a" is subjected to direct control by the counterbalancing valve "c" thus causing a problem on fine regulation capability in controlling the brake gear when rendered operative.